The present invention relates to the communication of television signals having a television information portion and a digital data portion, and more particularly to a method and apparatus for converting such signals formatted for transmission using a first modulation scheme to a format suitable for transmission using a different modulation scheme.
Television signals are frequently transmitted together with digital data in satellite television and cable television ("CATV") distribution systems. For example, digital audio information can be provided in nonvideo portions of a video waveform in lieu of the analog video components of a standard television signal. Two such systems are disclosed in U.S. Pat. No. 4,563,702 entitled "Video Signal Scrambling and Descrambling Systems" to Heller, et al and U.S. Pat. No. 4,613,901 entitled "Signal Encryption and Distribution System for Controlling Scrambling and Selective Remote Descrambling of Television Signals" to Gilhousen, et al. These systems provide for digital stereo audio transmission in the horizontal blanking interval of a National Television System Committee ("NTSC") television signal.
The Gilhousen, et al patent referred to above also discloses the transmission of digital data within a television signal for access control purposes. In particular, a transmitted television signal includes scrambled analog video information, and various control data in a "control channel" portion of the television signal for use in providing signal access only to authorized subscribers.
Most cable television systems receive programming signals via satellite for distribution over the cable network. In order to receive and distribute the satellite signals, the cable operator must have a satellite receiving station located at the headend of the distribution system. Similarly, private cable systems, such as those serving an apartment building, hotel or institution as a closed user group, may receive programming from satellites, in which case the system is referred to as a satellite master antenna television system ("SMATV"). Both cable television and SMATV systems usually distribute television signals using amplitude modulation ("AM"). The transmission of television signals to the CATV or SMATV headend by satellite, in contrast, uses frequency modulation ("FM"). FM modulation is used on the satellite link for its greater resistance to noise and distortion.
Unfortunately, the requirements for transmitting digital data using FM techniques are not always compatible with the requirements for transmitting the same data using AM techniques. The audio and control channel data included in television signals transmitted via satellite is typically pulse amplitude modulated ("PAM") multilevel (e.g., two level) data. The optimum PAM amplitude for two-level data differs widely between FM and AM transmission. Therefore, the CATV or SMATV distribution of a television signal received from a satellite is not a simple matter of demodulating the received FM signal to baseband, and then AM modulating the signal for distribution via cable. Modifying the signal at the satellite transmitter for subsequent AM distribution is not practical, since the satellite signal may also be provided to direct broadcast satellite ("DBS") subscribers, who receive television programming directly from a satellite using a small and low cost earth station they own and operate. In DBS applications, there is no AM retransmission of the television signal, and the data format must therefore be optimized for the FM satellite transmission.
It would be advantageous to provide a method and apparatus for translating television signals formatted for transmission using a first modulation scheme (e.g., satellite FM) to a format suitable for transmission using a different modulation scheme (e.g., CATV or SMATV AM). In particular, it would be advantageous to provide a format translator that adjusts the amplitude of data portions of a received satellite television signal to an acceptable level for communication using an AM modulation scheme. It would be further advantageous to provide such a format translator that is relatively simple in design and construction, reliable, and low cost.
The present invention provides a format translator having the above-mentioned advantages.